1. Field of the Invention
The present invention relates to a device for coding/decoding image information and, more particularly, to a device for coding/decoding image information, which is derived to determine the DCT coefficients of a block that is converted to an exterior block from a boundary one during a boundary block merge by means of merged information, and to perform a prediction of the DCT coefficients of the peripheral blocks by using the determined DCT coefficients.
2. Discussion of Related Art
When each pixel of image signals is represented by digital data of predetermined bits (for example, 8-bit data, 16-bit data, etc.), information content becomes excessively increased to communicate through public networks, and great costs are required to send/receive the large information content by a network or store it in a memory such as CD-ROM, deteriorating image quality. It is thus necessary to compress image information so as to send/receive image information via the conventional networks with a cost reduction.
As shown in FIGS. 1A-1F, the image information, to be compressed, is divided into classes. A video sequence layer comprises a plurality of GOPs (Group of Pictures) (See. FIG. 1A) each of which has I, B, and P pictures (See. FIG. 1C). One picture has a plurality of slices (See. FIG. 1B). One slice has a plurality of macro blocks (See. FIG. 1D) each of which includes 16.times.16 pixels. Four luminance sub blocks B1-B4 and eight chrominance sub blocks B5-B12 make one macro block, and each sub block has 8.times.8 pixels.
FIG. 2 shows a picture of an object having a predetermined shape that is made up with macro blocks (16.times.16 pixels) to be used in an image coding. As shown in FIG. 2, the macro blocks are classified into three types: interior macro blocks 22 having only interior information of the object, exterior macro blocks 23 having no interior information, and boundary macro blocks 21 partly having interior information.
In an intra mode where an image coding technique is applied to the macro blocks in coding the source picture instead of error signals, the sub blocks (8.times.8) are transformed by a discrete cosine transformation, and DC coefficients are quantized and predicted. DC prediction may be accomplished by a no loss coding where the blocks and DPCMs are applied in zigzags, as shown in FIG. 3. The white block indicates an interior block, the gray one a boundary block, and the black one an exterior block.
To code a DC coefficient, the quantized DC coefficient DC.sub.-- B4 of the sub block B4 of the macro block MB1 that is adjacent to the sub block B of the macro block MB2 to be coded is defined as the predicted value DC.sub.-- P of the sub block B of the macro block MB2.
As shown in FIG. 4, with respect to a block X to be coded, the previous block A, the left upper-sided block B, and the upper-sided block C are designated. The absolute value of the vertical gradient for the DC coefficients of the previous and left upper-sided blocks A and B is compared with that of the horizontal gradient for the DC coefficients of the left upper-sided and upper-sided blocks B and C. If the former is less than the latter, the DC coefficient of the previous block A is defined as a DC pre-estimated value that will be used to code the DC coefficient of the current block X. If the former is larger than the latter, the DC coefficient of the upper-sided block C is used as a DC pre-estimated value for coding the DC coefficient of the current block X. The differential, to be coded, can be obtained by subtracting the DC pre-estimated value from the DC coefficient of the current block X.
As shown in FIG. 5, in order to pre-estimate an AC coefficient, the first row coefficient (from the upper-sided block C) or the first column coefficient (from the left-sided block A) of the previous block, determined in the same direction as the DC prediction in FIG. 4, is used as the first row or column coefficient of the current block X in a differential coding.
During the prediction of DC and AC coefficients as described above, the sub blocks positioned inside the object or on the boundary of the object in a macro block can be scanned and predicted, while those out of the object cannot be coded.
As shown in FIG. 6, each boundary macro block has three types of sub blocks, interior sub blocks 61, exterior sub blocks 63, and boundary sub blocks 62. Two boundary sub blocks 62 and 64 can be merged so as to enhance a coding efficiency.
However, after a BBM (Boundary Block Merge) technique, the boundary sub block 4 as shown in FIG. 7A becomes the exterior sub block in FIG. 7B.
With the BBM technique and prediction of DC and AC coefficients, there is no appropriate treatment for the sub blocks transformed from boundary sub blocks to exterior sub blocks. If these sub blocks are treated as boundary sub blocks, they will require an unnecessary process for discrete cosine transformation and cause a deterioration of image quality as well as data errors during a data transmission.